A. Field of the Invention
This invention involves an electroplating bath and an electroplating process, along with a process for preparing the bath and a process and apparatus for regulating the metal content of the bath. More particularly, it involves processes for preparing, regulating and depositing a near eutectic tin-bismuth alloy. The invention also relates to the bath used for implementation of the electroplating process and the apparatus used for regulating the relative concentrations of tin and bismuth ions in the bath.
B. Description of the Prior Art
The electrodeposition of tin and various tin alloys is well known as a means to produce a solderable coating on printed circuit boards used for the interconnection of electronic components.
The complexity of printed circuit boards has evolved in response to the striking escalation in chip density. To achieve the required level of complexity, the feature sizes on printed circuit boards have been reduced to near 4 mils while the number of 0.006 inch (6 mils) thick inner-layers has increased to as many as 50. This means that the thickness of high technology circuit boards now approach 0.03 inches (0.300 mils). The finished copper plated hole size on these boards approaches 0.015 inches. Therefore, the board thickness to hole diameter ratio, or aspect ratio, is now near 20:1.
High aspect ratio printed circuit boards cause a number of unique problems in connection with the selection and uniform deposition of a solderable coating.
First, the copper within the high aspect holes can be damaged if the reflow soldering temperature is too high. This damage results from the mismatch in thermal expansion between the copper in the plated through hole, (PTH), and the insulating glass reinforced plastic materials of the printed circuit board.
The melting point of normal tin lead solder is 183.degree. C. and processing temperatures are typically between 220.degree.-250.degree. C. Processing boards within this temperature range can excessively strain the copper in the PTH. The z-axis expansion, because it is not tempered by glass reinforcement, can result in stress and deflection of the copper plating causing the copper barrel to break.
Second, the higher the temperature at which the soldering process takes place, the longer the time the process takes. This is due to the fact that during the soldering process the entire printed circuit board must be heated to the process temperature. For small printed circuit boards this extra time is not significant. However, large, multilayer printed circuit boards have significant mass which must be heated to the process temperature and the time involved can unreasonably slow processing and permit oxidation and intermetallic growth.
Finally, the solderable coating must have good throwing power in order to effectively and uniformly plate the high aspect ratio holes associated with very thick printed circuit boards. The throwing power of a plating bath is a measure of its ability to uniformly plate the interior surface of a hole. Poor throwing power results in inadequate plating of the interior of holes which ultimately results in defective printed circuit boards or high board failure rates.
Solderable coatings of tin or tin alloys currently used are not effective in solving the soldering and process problems associated with plating thick multilayer printed circuit boards. It is well known that the addition of small amounts of bismuth to tin or tin alloy is advantageous in inhibiting "tin pest" and in reducing the formation of "tin whiskers". Tin pest is a condition whereby the metallic properties of tin are degraded at low temperatures. A loose tin power is formed which separates from the base metal thereby exposing the underlying metal to the effects of corrosion. Tin whiskers is a descriptive name given to the formation of whiskers of tin which develop on the surface of the deposit and can cause electrical shorts and related problems. See, for example, U.S. Pat. No. 3,663,384 to Lescure, U.S. Pat. No. 4,162,205 to Wilson, U.S. Pat. No. 4,252,618 to Grenda, U.S. Pat. No. 4,331,518 to Wilson and U.S. Pat. No. 4,565,610 to Nobel et. al. These patents all disclose the use of very small amounts of bismuth in the solderable coating, the concentration of which is significantly less than the concentration of tin. These patents do not teach or suggest the use of a significant concentration of bismuth to obtain a solderable coating with a low melting temperature and high throwing power. For example, U.S. Pat. Nos. 4,331,518 and 4,162,205, both to Wilson, disclose plating processes resulting in from 0.15% to 0.80% by weight of bismuth in the electroplate. U.S. Pat. No. 4,252,618 to Grenda discloses the presence of from 0.08% to 1.1% by weight of bismuth in the alloy. These levels are used to solve the problems resulting from, tin pest and tin whiskering, but do not result in any improvement of solderability, or appreciable reduction of melting temperature.
In order to solve the soldering and process problems discussed above it is necessary to provide a plating bath and plating process with high throwing power capable of depositing a solderable coating with a melting point less than presently used with tin-lead solders. It is known that the deposition of a eutectic tin-bismuth allow consisting of 43% by weight tin and 57% by weight bismuth will result in a solderable coating with a lower melting point. However, no commercially feasible tin-bismuth plating baths or plating processes exist which consistently produce a uniform deposit of tin-bismuth alloy at near eutectic composition both on the surface of the printed circuit board and in the interior of high aspect ratio holes. Therefore, a need exists to provide an electroplating bath, method for regulating the bath and method for plating which may be regulated in a manner that allows the plating process to be commercially feasible.